There is considerable interest in hydrogen as a replacement for fossil fuels because of its high energy density per unit weight, its readily availability through the electrolysis of water, and the absence of polluting byproducts from its use. A number of technological components represent challenges in making this transition from fossil fuels to hydrogen, and in the development of appropriate systems and infrastructure that can integrate into those that already exist. A particular challenge of the developing hydrogen economy and the automotive industry thus involves the current paucity of fully satisfactory systems for hydrogen storage, ones that are safe, reliable, conformable, light-weight, and comprehensively economic. The technological issues underlying various approaches to hydrogen storage include the form within which hydrogen is stored, the nature of the medium holding the hydrogen, and the operation of the containers holding the medium.
Holding hydrogen as a compressed gas and as a cryogenic liquid are the most traditional forms of hydrogen storage. Compressed hydrogen can be stored in high pressure tanks (up to 10,000 p.s.i.). A problem with this method is that hydrogen diffuses very effectively, particularly when under high pressure, and currently available high pressure tanks do not effectively prevent such diffusion over an extended period of time. The requirement that tank materials be lightweight, and the fact that tank failure or damage in the event of an accident would be catastrophic provides further reason for pause. Storage of hydrogen in liquid form, at cryogenic temperatures is also an approach that shows some promise.
Metal hydrides, such as magnesium-based alloys, have also been used as media to store hydrogen. Although this method does not require high pressure and is operable at room temperature, there are other drawbacks. Metal hydrides are heavy, generally heavier than the hydrogen gas by a factor of about 50. Metal hydrides also undesirably contaminate the hydrogen as it is released. Further, metal hydride storage is not energy-efficient in this context; the energy required to extract the hydrogen from the metal hydride is equivalent to nearly half the amount stored within it. Metal hydride storage has been disclosed by Liu et al. (U.S. Pat. No. 4,358,316), by Bernauer et al. (U.S. Pat. No. 4,446,101), and by Ovshinsky et al. (U.S. Pat. No. 6,328,821).
Activated carbon has been used to store hydrogen at cryogenic temperatures and moderate pressures (50-70 bar), as has been described by Schwarz (U.S. Pat. No. 4,716,736). Cryogenic storage in activated carbon can be done at a 80K, a temperature higher than that required for liquid hydrogen storage. Hydrogen can sorb to surfaces in the activated carbon and can be released by increasing the temperature. Commonly available activated carbon, however, is not very pure, and contaminants are released with the hydrogen. Many researchers have found that it is difficult to obtain release of all hydrogen stored on activated carbon. Other problems associated with activated carbon include low weight percent storage capacity and the need to maintain cryogenic temperatures. Some of these problems have been discussed by Hynek et al. 1997, in “Hydrogen storage by carbon sorption,” Int. J. Hydrogen Energy, 22, No. 6, 601.
Alternative forms of carbon for hydrogen storage that are being explored include carbon nanotubes and graphite fibers, which, according to Rodriguez et al. (U.S. Pat. Nos. 5,653,951 and 6,159,538), sorb hydrogen by chemisorption. Another approach using nanostructured materials, in this case comprised of light elements, is one in which hydrogen is bound by physisorption, as described in pending patent applications of Bradley et al. (U.S. Application No. 60/020,392) and Kwon et al. (U.S. Application No. 60/020,344). Other carbon-based approaches include the utilization of turbostratic microstructures, as described by Maeland (U.S. Pat. No. 6,290,753) and aerogel preparations of carbon fullerenes, as described by Lamb (U.S. Pat. No. 5,698,140). While structured forms of carbon offer advantages over non-structured activated carbon, the basic thermodynamic properties of carbon which determine the low operating temperatures at which hydrogen is desorbed from the medium remain the same.
Hydrogen storage and supply systems that operate at higher temperatures and lower pressures than those currently available are an important and as yet undeveloped component of the developing hydrogen economy. Porous storage media represent a realistic approach to the development of such systems, and thus there is an acute need for materials which offer high capacity and favorable operating conditions of temperature and pressure. It is further desirable that such media be manufacturable economically, and at industrial scale.